Fade in action applicable to all objects (in cycles)
You could use a node group to do this:


Add a mix shader and create a new node group by selecting it and pressing CtrlG
Add a transparent shader to the node group:


Add this node group at the end of all the materials you want to simultaneously fade in/out:



This process can be made a little bit faster with copy and paste:


Select the node group and press CtrlC
Select each object/material and press CtrlV to paste the node group.

Adjusting the Factor of the mix node inside the node group will set it for all instances of the node group


With the factor set at .5 and .8 (click for larger image):